Myocarditis is a major cause of sudden death in people under 40 years of age, and many of these cases are associated with an autoimmune process[1, 2]. Like other autoimmune diseases, the fundamental causes and mechanisms of pathogenesis of myocarditis are not understood. To study the mechanisms of autoimmune myocarditis and autoimmune diseases in general we have developed a murine model of experimental autoimmune myocarditis (EAM) induced by cardiac myosin [3]. This model demonstrates that there are strong genetic influences to susceptibility to myocarditis, offering a fresh avenue into understanding the pathogenesis of this autoimmune disease [8]. The EAM model is unique and worthy of study apart from other autoimmune disease models because it shows greater influence of non H-2 genes, and shows an unusual male influence. In preliminary work we have demonstrated that loci on murine chromosomes 6 and possibly 1 and 4 are involved in susceptibility. Two of these loci (Chr. 1, Chr.6) interact and are also implicated in other autoimmune disease such as diabetes [7]. Furthermore, the Chr. 1 locus includes CTLA-4, an immunologically important gene, which we have previously demonstrated to regulate the pathogenesis of imyocarditis. The Chr.6 locus, which functions primarily in males, overlaps with loci that are important in thymocyte homeostasis and apoptosis [5]. We propose to conclusively establish the genetic findings which will be the foundation of future positional cloning and perform functional studies of polymorphisms in CTLA-4 which may lead to differential susceptibility to myocarditis. Preliminary experiments also indicate that these genetic loci act through hematopoietic tissues. We propose to solidify these findings and investigate through which specific cell types in the immune system these genetic loci operate. We build on our preliminary data to propose hypothesis-driven aims to identify host genes that control susceptibility and characterize their mechanisms of action in a murine model of autoimmune myocarditis.